There is a broad range of analytical techniques which rely on light interaction with a particular medium, as for example visible absorption spectroscopy (VIS spectroscopy), surface photovoltage spectroscopy (SPS), or scanning light pulse techniques (SLPT or LAPS) used in chemical or biochemical sensing.
All of them provide useful and complementary information of the target environment (hereinafter sometimes referred to as an “analyte”) but due to the complexity of the systems, which are mainly related to the requirements for the light sources, their extension to practical applications is hampered, being in general confined to expensively furnished laboratories or to devices dedicated for specific purposes.
The complexity and sophistication of the involved light sources comprised in these experiments, relate to the optical components and the controlled micro-positioning required to direct the monochromatized light through the output slit.
The objective of the present invention is to provide a practical low cost method and associated devices which supplies micro-positioning, monochromatization and intensity modulation allowing to exploit benefits from the above mentioned techniques and others requiring precisely controlled light sources.
The invented method in general terms, consists of the use of a program controlled display (like computer monitors, mobile telephones or TV screens) as a light source. The mechanical positioning is substituted by the ability of the screen to sequentially illuminate contiguous pixels, precisely and regularly patterned in any of the different alluded displays. Instead of a monochromator this same accurate positioning can be used to monochromatize light using a illuminated region on the screen as a mobile light source in front of a fixed diffractive element specially placed in front of the light.
Additionally, different RGB colors just sequentially displayed in the screen provide a light source suitable with spectral response measurements, which is a main goal for colorimetric sensing approaches. Furthermore the different pixels in the light source can be individually programmed with respect to color and intensity. Through the use of a lens between the light source and the test object also small test objects can be illuminated. The use of a program controlled display as a light source makes it also possible to scan the light over the test object.
The use of detectors like digital cameras, video and web cameras is thus one interesting possibility.
The invention allows for instance simplified illumination of a test sample and/or detector specially designed to be affected by a target environment. In this respect already developed indicator materials or molecules for optical detection using normal (monochromatic) light sources can be used. It is also the possibility to develop indicators which are optimized to be used together with rgb-colors.
Additionally, the information can be processed in situ by the user or only acquired in situ but analyzed on line through an internet connection, where expert interpretation can be supplied.
The general inventive principle mentioned above, has particular characteristics depending on the considered analytical technique that is to be emulated.
For instance, in the case of VIS spectroscopy, the standard technique requires a visible light source, a monochromator, a chopper and a detector.
The monochromatized light, with a narrow spectral width determined by the monochromator slit width, is passed through a transparent cuvette where the sample substance is placed, in this case a liquid, and the emerging light is captured in a detector. Depending on the detector requirements a light chopper can be interposed in the light path (FIG. 1).
The VIS absorbing properties of different materials depend on their composition, and by the introduction of chemical agents (like chromophores or fluorescent labels) a large amount of chemical or biochemical properties can be traced and selectively identified through their spectral response.
The principle is not only limited to liquids, but also absorbing solids, gels or polymers like labeled DNA array slides or gas sensitive polymers can be analyzed.
Spectra acquisition requires a computer controlled system able to coordinate the supplied wavelength and this minimally comprises a programmable monochromator constraining the technique to laboratories.
In the case of SPS, or similar techniques used in semiconductor interface analysis like electric field induced SPS (EFISPS) or internal photoemission spectroscopy (IPE), the main principle is to excite carriers in semiconductor structures which are illuminated at a particular wavelength. The spectral range to scan depends on the semiconductor band gap but also sub-band gap energies can provide information of surface states.
In SPS, a focused monochromatic light beam is used to illuminate the semiconductor substrate at a controlled chopping frequency, which provides transient photocurrents related to surface or interface states (a complete review of SPS and related techniques can be found in L. Kronik, Y. Shapira, Surf. Sci. Reports 37, 1–206 (1999)). The technique, can be adapted for sensing applications, but still requires an expensive controlled light source (FIG. 2).
If the light beam or a laser beam is scanned over the semiconductor, a spatially resolved map of the interface properties can be composed in the so called SLPT techniques (I. Lundström, et al., Nature 352, 47–50 (1991)). Providing chemically sensitive biasing electrodes of different materials and thicknesses spatially distributed the technique provides selective chemical images to gas mixtures and odors (FIG. 3).
If the biasing electrode is replaced by an electrolyte the device becomes a powerful potentiometric tool for chemical or biochemical analysis known as light addressable potentiometric sensor (LAPS, D. Hafeman, et al., Science 240, 1182–1185 (1988)).
In many of these approaches again, there are complex elements as micropositioned modulated laser beams with the associated focusing optics which makes field applications impracticable.